The present invention relates to a method of controlling the ignition timing of an internal combustion engine.
A well-known method of controlling the ignition timing of the engine consists in detecting the flow rate of the intake air or the pneumatic pressure in the intake manifold and the rotational speed as operating condition parameters, calculating an optimum ignition timing from the detected operating condition parameters by using a predetermined function which represents the relationship between optimum ignition timings and the operating condition parameters, and practically adjusting the ignition timing responsive to the calculated value. The optimum ignition timing should be set so that the best torque can be obtained without development of knocking under any operating condition. In general, however, the lower timing advance value of the knocking zone changes with the intake air temperature. Therefore, it is very difficult to select the optimum ignition timing at which best torque of the engine can be obtained without developing and knocking. Namely, when the intake air temperature is low, the lower timing advance value of the knocking zone changes toward a more to advanced ignition timing. When the intake air temperature is high, on the other hand, the lower timing advance value changes toward a more delayed ignition timing. Therefore, if the ignition timing is selected to be advanced as much as possible so that best engine torque is obtained without developing knocking at a low intake air temperature, a rise in the intake air temperature then causes knocking which deteriorates the operation feeling and produces harmful gases. On the other hand, if the ignition timing is selected to be advanced as much as possible so that best engine torque is obtained without developing knocking at a high intake air temperature, an excessive margin is provided against the development of knocking, decreasing engine torque is decreased and increasing fuel consumption, when the intake air temperature is low.
The above problem occurs in an engine in which the ignition timing for producing the best torque (hereinafter this timing is referred to as MBT) lies in the knocking zone, and thus knocking develops at an ignition timing which lags behind MBT when the ignition timing is advanced. In other engines or even in the same engine, however, the MBT may appear on the side lagging behind the knocking zone under light to medium load conditions. With this type of engine or under this load condition, the spark advance angle for generating the MBT will shift toward the delay direction when the intake air temperature is low, and shift toward the advance direction when the intake air temperature is high. Therefore, if the ignition timing is selected so that a best engine torque is obtained at low intake air temperature, the ignition timing considerably lags behind the MBT causing the obtained engine torque to be lowered and fuel consumption to be greatly increased when the intake air temperature is high,